COPR is produced during the process of extracting chromium (i.e., chromite (Cr2O3)) from its ore. The extraction process involves roasting the chromite ore to oxidize the chromium therein from the trivalent to the hexavalent state, then treating the oxidized chromium with soda ash (i.e., Na2CO3) to form sodium chromate (Na2CrO4). The principal reaction may be described as follows:Cr2O3+Na2CO3+1.5 O2→2Na2CrO4+CO2
Lime (i.e., CaO) is added during the roasting process to act as a mechanical separator, allowing air circulation within the ore and thereby promoting the reaction of oxygen with chromite and sodium carbonate. Lime also serves as a sequestering agent; combining with various impurities present in the ore to form insoluble complexes. The sodium chromate formed during the roasting process is then extracted with hot water as weakly-colored yellow liquor. The sodium chromate is then converted into sodium dichromate (Na2Cr2O7) by reaction with sulfuric acid. After draining, the residue is discarded. The disposed COPR contains unreacted chromite ore and residual chromate, and has a high alkalinity due to the soda ash and lime used in the roasting process.
COPR was used as fill material at various locations in the United States, but was discovered to not be as benign as initially thought. In particular, yellow chromate solution was observed to leach from locations where COPR had been deposited, and structures built on sites where COPR had been used as fill experienced catastrophic failures due to the heaving and uncontrolled expansion of the COPR material.
In the environment, chromium exists mainly in two oxidation states: hexavalent chromium (Cr(VI)) and trivalent chromium (Cr(III)). Hexavalent chromium is highly mobile, acutely toxic at moderate doses, and classified as a known respiratory carcinogen in humans. In contrast, trivalent chromium is a dietary element, not classified as a carcinogen, and is immobile in most environmental settings. Most of the known stabilization methods involve the chemical reduction of hexavalent chromium (i.e, Cr(VI)) to trivalent chromium (i.e., Cr(III)) concurrent with pH adjustment. The reductants typically used include elemental iron, pyrite, ferrous compounds, organic compounds, and sulfide compounds.
In response to the known toxicity of hexavalent chromium, the United States Environmental Protection Agency (“USEPA”) has set a target level for chromium in leachate of 5 mg/L for in situ treatment of matrices contaminated with chromium, as determined by the Toxicity Characteristic Leaching Procedure (“TCLP”; USEPA Method 1311, Test Methods for Evaluating Solid Waste, Physical/Chemical Methods, EPA Publication SW-486). The USEPA has also set a maximum leachate concentration of 0.6 mg/L (as determined by the TCLP method) for wastes that are to be identified as non-hazardous under the Resource Conservation and Recovery Act (“RCRA”) and thus suitable for disposal in “non-hazardous” (Subtitle D) landfills.
The leaching of chromate from a chromium-contaminated matrix can be substantially decreased through stabilization. Chemical stabilization of COPR is inhibited by the high alkaline content of the COPR matrix and the slow release of chromium and alkalinity from the COPR minerals. Geotechnical stabilization is inhibited by the potential swelling of the COPR matrix, before or after treatment, as it releases alkalinity.
A survey of the state of the art of COPR remediation revealed that present practices are generally unsuccessful at achieving the USEPA target levels; or that they can achieve such target levels, but at prohibitively high costs; or that they have severely limited applicability in the field. U.S. Patent Publication No. U.S. 2004/0126189 A1 discloses a process of (1) generating a solution of ferrous sulfate and sulfuric acid by oxidation of iron pyrite, then (2) percolating the acidic solution through a column of COPR to reduce hexavalent chromium to trivalent chromium, while (3) oxidizing the dissolved iron to its ferric state. However, to effectively treat the lower portions of the COPR column, it would be necessary to consume the entire alkalinity of the COPR material, in the upper portions of the column requiring that the quantities of the acidic solution added to the upper portions of the column be far greater than is needed to neutralize the alkalinity present. It is also noteworthy that the ferrous or ferric ions would precipitate from solution as iron hydroxides in zones of high alkalinity. Not only would such precipitation consume the dissolved iron, but the precipitate would inhibit the flow of the acidic solution into those zones. Further, the acidic ferrous solution would flow preferentially through certain pathways in the column, with the result that materials outside of such pathways would remain untreated.
U.S. Pat. No. 5,202,033 discloses an in situ method of treating soil or solid waste contaminated with chromium, by introducing a solution of ferrous sulfate and a substance for use in pH control. This method would present the same difficulties as were discussed above with regard to U.S. Patent Publication No. U.S. 2004/0126189 A1.
U.S. Pat. No. 5,304,710 discloses an ex situ process in which ferrous sulfate is used to chemically stabilize COPR. In this process, the COPR matrix is conditioned by reducing its particle size to 100 mesh, and acidifying it to a pH of 3. Ferrous sulfate is then added to reduce hexavalent chromium to its trivalent state. Finally, the pH of the COPR material is raised by the addition of lime to precipitate and immobilize the freshly formed trivalent chromium. An inherent weakness in this treatment scheme is that it consumes amounts of acid, ferrous sulfate and lime that are far in excess of the theoretical quantities needed to condition the COPR, reduce the hexavalent chromium and stabilize its trivalent form.
Other references disclose the use of solidifying agents to further improve the effectiveness of the chemical stabilization achieved by treatment with ferrous sulfate. U.S. Pat. No. 5,285,000 discloses the use of sodium silicate gel to form a relatively impermeable matrix for chromium-contaminated soil that had been treated with ferrous sulfate. However, this treatment was not tested with COPR, which is more recalcitrant than a soil matrix.
U.S. Pat. No. 5,397,478 describes the use of phosphates as bonding agents after treating chromium in solid wastes using any of a variety of reductants. Examples are provided for the use of sodium dithionate, and include TCLP data from samples that had been cured for a few hours. The reference does not address the long-term stability of the treated wastes.
U.S. Pat. No. 3,981,965 discloses the use of calcium sulfide and sodium hydrosulfide to reduce chromate in chrome waste residue. The effectiveness of the disclosed treatment method was evaluated by the passive method of exposing a pile of treated material to natural precipitation, and collecting the runoff from the pile for analysis. Such a test method does not meet the standards of aggressive testing (e.g., the TCLP protocol) that are now required by the USEPA, and, therefore, cannot be relied upon as evidence that the disclosed treatment method is effective to achieve the target levels that are currently applied.
Other methods of stabilization include mixing blast furnace slag with mud or sludge to prevent leaching of chromium from COPR. For instance, U.S. Pat. No. 4,504,321 discloses that mixing COPR with finely ground granulated blast furnace slag and mud or sludge dredged from various water environments will reduce hexavalent chromium to its trivalent state and form a highly impermeable matrix suitable as stable load-bearing landfill material.
U.S. Pat. No. 6,221,002 discloses the use of ascorbic, acetic, or citric acids for the remediation of chromate bound in COPR material. However, such acids may form aqueous complexes with trivalent chromium, causing the treated material to fail the TCLP test.
U.S. Pat. No. 5,562,588 discloses that mixing the COPR matrix with organic material containing bacteria and nutrients is effective as an in situ treatment for COPR. However, high concentrations of chromium in the leachate liquor will be toxic to the microbial consortia, confining the effectiveness of this treatment method to soils or mineral matrices having lower concentrations of chromate.
U.S. Patent Publication No. 2004/0086438 A1 discloses a method of recovering salts of hexavalent chromium by heating COPR to 350° C. with metal hydroxide, then leaching the chromate thus formed in hot water. Iron salts are recovered by treating the residue with hot mineral acid, followed by addition of a base. This is an energy intensive process, and it does not address the swelling of the treated residues after treatment.
U.S. Pat. No. 5,967,965 discloses the removal of hexavalent chromium from contaminated soil by successive soil washing steps using solutions of anionic or cationic synthetic organic flocculants. The chromium in the recovered floc is chemically reduced, and treated to promote the alkaline formation of insoluble metal hydroxides.
Previous attempts to chemically stabilize the COPR matrix, such as those processes disclosed in the references discussed above, have been only temporarily effective in inhibiting chromate leaching to concentrations below regulatory levels. Factors contributing to the eventual failure of such processes include the back-conversion of trivalent chromium to hexavalent chromium, and the propensity of the COPR material to swell, thus disrupting the treated matrix. An exhaustive investigation and evaluation of commercially available soil treatment technologies showed that none of the known methods addresses the critical issues related to long term treatment performance. It should also be noted that present practices of acid application to reduce the alkalinity and high pH of the COPR may not be effective in the field. Further, the infeasibility of applying strong acids, as currently practiced, is exacerbated by the strongly exothermic nature of their reaction with the alkaline COPR.
None of the technologies that were reviewed address the swelling of the COPR material before or after treatment, which is the cause of heaving, or the mechanisms that are responsible for the production of the minerals responsible for such swelling. Moreover, exhaustive literature searches and review did not yield any information on mechanisms of chromate leaching or on the long-term release of chromate from the COPR matrix. The investigations underlying the present invention have identified potential mechanisms of chromate release, and of the formation of swell-inducing minerals, allowing the development of effective treatments for long-term control of chromium release and expansion of the COPR matrix.